Recommended 1000W motor for electric bicycle

Discussion in 'Homework Help' started by zazas321, Apr 18, 2018.

  1. zazas321

    Thread Starter Member

    Nov 29, 2015
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    Hello guys. I have a task to pick a AC motor that would be able to run electric bicycle at 20mph constant. I need to pick a random scenario and calculate the power requirements and choose the best suitable motor available and design a system on matlab.

    I have done the calculations for power requirements that I would need arround 1000w of power to be able to keep a constant speed of 20mph on a 5 degree slope with a 70kg rider and 5 mph wind speed. I have done all the friction, gravity and air resistance calculations and ended up at arround 950w power requirements. Other forces can be neglected.

    I am not sure what would be the next step. How do I pick the most suitable 1kw motor and calculate required amount of rpm and torque to produce 20mph?
     
  2. ericgibbs

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  3. AlbertHall

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    The resulting bike would be a bit of a problem in the UK.

    http://www.cyclingweekly.com/news/product-news/electric-bikes-uk-law-234973
    What if my electric bike breaks the rules?
    If your ebike doesn’t meet these regulations – either because the motor is more powerful than 250W, or if it assists you when you’re riding more than 15.5 mph – it will need to be registered, insured and taxed as a motor vehicle. In this case, you will also need a driving licence, and you must wear a motorcycle helmet.

    These other kind of bikes (also called speed pedelecs) cannot be ridden on cycle paths and must be approved by the DVLA. So while it’s easy to de-restrict an ebike to get the motor assisting you with higher speeds, it is not very wise to do so, both for regulatory and safety reasons.


    Read more at http://www.cyclingweekly.com/news/product-news/electric-bikes-uk-law-234973#oobLI35FjF4ELPgO.99
     
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  4. Alec_t

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    Sep 17, 2013
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    Considering the similarity to that other thread in the post #2 link, is this a college assignment?
     
  5. Janis59

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    Aug 21, 2017
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    My experience with those are that e-bay has hundreds of offers what differs with voltage and wattage, but only real difference is do they are hang-on engines what You connect in the wheels by means of surplus chain, or them are inner drum, which You spoke-in the rim, so the any chain is unnecessary.
    Before You have choose one of those both contradicting ways, there is no way to provide an another hints.
    Only I can comment just now, the 1kW is VERY much. Basically for application in hardly mountainy region or on drastic mud-roads. The European law states the 200W and below needs no a driver licence, road tax, insurance and technical inspection taxation. Thus 200W and under is huge benefit, and it is anyway 4-fold stronger than average non-sportsman foot.
    As You ought know, the 60W is whole-day-long work of average bicycle tourist, 120W is load what gives immediate sweat on the forehead, 240W is power what well trained body may give for short distances, under the one minute long, and 1 kW is maximum maximorum 5 seconds long for 99,9% of man, except the World record for pedal-driven aircraft flying over La-Manche of 15 seconds long effort of 3 kW. But that guy had trained every day for some 5 years long and failed many times before once won.
     
  6. zazas321

    Thread Starter Member

    Nov 29, 2015
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    Yeah I know that 1kw is much higher than the UK law allows. But i do not need to build or even prototype it. Im just asked to pick a sccenario and calculate power requirements. Then pick ac motor that would be able to run the bicycle and simulate everything on matlab. That is the task. I can reduce the slope and end up with lower power requirements ( arround 750) but there is no way to keep 20mph opposing wind and friction forces with 250w. That is not important cause as I just said; this is not a real thing these are just calculations and simulations
     
  7. zazas321

    Thread Starter Member

    Nov 29, 2015
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    Lets say i have calculated that the total forces that cyclist needs to oppose are 85N. Is it correct to say that the Torque required will be T=F*r
    r=radius of the wheel. Consider 26 inch wheels (0.33m) Therefore T=85*0.33=28Nm. Does that mean that in order to maintain 20mph speed motor would need 28Nm force at 750W ?
     
  8. Tonyr1084

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    Sep 24, 2015
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    Again, is this a college assignment?
     
  9. zazas321

    Thread Starter Member

    Nov 29, 2015
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    Yes. Does that change anything?
     
  10. bertus

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  11. Tonyr1084

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    Sep 24, 2015
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    It'll probably move this thread to homework help or something like that. I'm not a moderator so I don't, can't and won't change anything about it. However, this is my thoughts on the matter; your assignments are for your learning experience. We may help you arrive at the correct conclusion but I doubt anyone here wants to get an A for you. It already sounds to me like this is over my head so I can't help. But it's only fair to let others know the reason for your question. I ask questions all the time. Sometimes it's help fixing something. Sometimes it's to discuss ideas. Sometimes it's to find an alternative way of achieving a goal.

    Homework help is just as valid when asking for help. If there's something you don't understand then by all means - ask. But there have been people come here to ask for us to engineer something FOR them. Meaning they didn't want to do the work. And that's just not fair to us. We don't get paid to do this. The payment we receive is in the form of gratification knowing we've been able to help someone solve a problem. But helping someone with a problem and solving it FOR them are two different things.

    [edit] Bertus moved it 2 minutes before my post.
     
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  12. shortbus

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    Sep 30, 2009
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    Aren't you forgetting the gear ratio between the motor and drive wheel? That is what allows a smaller motor to drive a larger weight, in any vehicle.
     
  13. zazas321

    Thread Starter Member

    Nov 29, 2015
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    Yes I am not expecting anyone to solve the problem for me. Il give some more detail:
    Weight of the cyclist (including bicycle) (M) = 70 kg
    Speed of cyclist = 20mph
    Wind speed = 5mph
    Drag coefficient (Cdrag) = 1
    Friction Coefficient (Cfriction) = 0.008
    Slope 5 degrees.
    So assuming that the cyclist needs to oppose Forces of friction, wind drag and gravity, according to my calculations I end up with the Total force of 111N.
    To conver Total force to Power, I have multiplied Total Force by required cyclist speed (20mph) and got about 970W.
    My task required to select a motor that could deliver enough power so that the bicycle would be able to go at a constant 20mph on a 5 degree slope.
    I have calculated The torque required using the formula T=F*r. Assuming 26inch wheels (0.33m radius), the torque required is 36Nm. Do my calculations make sense?
    Assuming the formula Power (kW) = Torque (N.m) x Speed (RPM) / 9.5488

    I can calculate what speed I would require having 1000w motor and 36Nm . Speed=Power/Torque * 9.5 =260RPM

    So I need 1000w motor with 260RPM and 36Nm torque. Does that make any sense or I need to chose abit more realistic situation? For example reduce the slope

     
  14. shortbus

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    Sep 30, 2009
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    The gear reduction will still be needed. A 26inch wheel running at the motor speed will go a lot faster than 20MPH. You need to divide the diameter of the wheel into a mile distance. Gearing does two things, lets you use the motor at full rotational speed(full horse power) and lets you use a smaller motor. Think about it, 10,000 RPM motor times the diameter of the wheel, going to go a lot faster than 20MPH.
     
  15. zazas321

    Thread Starter Member

    Nov 29, 2015
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    But what about torque? How do I take torque into consideration of driving the bicycle?
     
  16. Tonyr1084

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    Sep 24, 2015
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    If you have a gear reduction of - lets make up some numbers here - a reduction of 10 to 1 (ten motor revolutions to 1 tire revolution) and you know your motor is capable of producing 10 foot pounds of torque at a given RPM then the torque at the wheel will be 100 foot pounds.
     
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  17. zazas321

    Thread Starter Member

    Nov 29, 2015
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    Okay I think im starting to understand. So If I need 260rpm and 36Nm torque. Consider 3000RPM 3.5Nm torque motor. If I have a gear reduction of 10 I would get 300RPM and 35 Nm which is close enough to my requirements.
     
  18. shortbus

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    Are you saying the bike has 26"wheels? Or is that the actual measurement of the out side diameter of the tire? On tires the 26" is the inside diameter of the bead, normally. A wheel with a 26" diameter would be closer to a 24" wheel.

    In my post #14 I said, "You need to divide the diameter of the wheel into a mile distance", I meant to say circumference not diameter. Here is a good site for things like your doing, a lot of engineering information on many different things in one place. https://www.engineersedge.com/mechanics_machines/transmission-gear-ratio-calculator.htm
     
  19. Tonyr1084

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    Sep 24, 2015
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    Assuming a 26 inch tire (26 inches tall) you have a circumference of (26 x Pi =) 81.68 inches of forward roll per one revolution. A mile has 5280 feet (63,360 inches). Your tire will turn (63,360 ÷ 81.68) 775.7 times per mile. At 60 MPH that's 775.7 RPM. [as you can tell, I'm using Imperial measurements]

    If your motor is turning 3,000 RPM then you need a gear ratio of (3000 ÷ 775.7) 3.87 to 1. Your motor has to spin 3,000 RPM with a gear ratio of 3.9 to 1 in order to travel 60 MPH. Now, if you want the speed of 20 MPH then that's 3 times slower than 60 MPH. You can increase your ratio of 3.9 to 1 - to - 11.6 to 1. Whatever HP your motor produces at 3,000 RPM you can multiply that by 11.6 to get your driven horse power. If your motor produces 5 HP at 3,000 RPM then you're theoretical bicycle is being drive at 58 HP. (11.6 x 5) This is assuming your motor pulley is one inch diameter and the drive pulley diameter is 11.6 inches in diameter.

    Assuming your peak HP is at 3,000 RPM a gear reduction of 11.6 to 1 will give you 20 MPH. Whatever the HP of the motor - THAT times 11.6 will give you the final output. If that's enough to move an average person up hill at 5 degrees incline against a given wind resistance then you're good to go. But honestly, if you're in college you should already know these numbers. This stuff is elementary, and even I know this stuff. And math is NOT my strong point.
     
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  20. shortbus

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    But for a V belt that is not a good thing. In real life the smallest pulley to use is around 2 inch. Even though they sell 1 inch. A 1 inch is hard on belts, too much flex, breaks down the cords in the belt. Chains, ribbed belts or timing type belts are OK though on small diameters like that.
     
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